Field of the Invention
The present invention relates to a lithographic apparatus. The present invention further relates to methods of manufacturing devices using lithographic apparatus calibrated by such a method, and to data processing apparatuses and computer program products for implementing parts of such a method.
Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
A key requirement of the lithographic process is to be able to position a pattern in the correct position relative to features formed in previous layers on the substrate. Alignment sensors are provided for this purpose. As device structures become ever smaller, alignment specifications become ever tighter. Similarly, at least in the case of optical lithography where the patterns are applied using an optical projection system, a key requirement is to measure accurately the local height of the substrate, so that the pattern can be optically focused onto the resist layer.
Known forms of alignment sensing arrangements are disclosed in published patent applications such as US2008/043212A1 (Shibazaki) and US 2011/013165A1 (Kaneko), both of which are incorporated herein by reference. To reduce the time taken for measuring many positions across the substrate, these known examples provide multiple alignment sensors, operable in parallel. To obtain a highly accurate position measurement, each alignment sensor should be focused on the substrate surface (or on a target mark beneath the substrate surface). However, since the substrate is generally not perfectly flat, it is impossible for all of the alignment sensors to capture focused images of several marks at the same time. In the published applications, the alignment sensors are operated to capture multiple images of the same marks, each time with a different height (focus) setting. The best measurement of each mark is selected from the image where the corresponding sensor was in best focus. While the known system can provide accurate position measurements, the time taken for the multiple measurements can cause a reduction in throughput of substrates in the manufacturing process.